This invention relates to a process for producing pulp from vegetable materials including a step for treating waste liquor generated in the pulp production to recover the chemicals therefrom for reuse.
There are a variety of chemical processes for producing pulp on mill scale and the two prominent processes among the prior art chemical processes for producing pulp are the so-called sulfite process and the so-called sulfate or kraft process. However, these prior art processes have inherent disadvantages such as (1) in the pulping stage of the prior art processes, a substantial portion of the hemicellulose and cellulose in the materials are lost and the fibers of the materials themselves are chemically and/or mechanically damaged resulting in lowering of the yield of pulp and also in reduction of the strength of paper prepared from such pulp; (2) as to the sulfite process, no pertinent technology for treating waste liquor generated in the pulping step has been established; (3) as to the kraft process, although a pertinent technology for treating waste liquor has been established, the process emits a particularly offensive smell; (4) both the prior art processes make it difficult to economically treat waste liquor generated in the bleaching step; and (5) both the prior art processes require the performing of the pulping and bleaching steps separately resulting in complication of equipment and operation procedure.
In view of the above-mentioned disadvantages of the prior art pulp production processes, efforts have been made to develop a chemical pulping process which can produce pulp with high yield and which can effectively eliminate environmental pollution which may be otherwise caused by waste liquor and exhaust gasses and which have been inevitable in the prior art pulping processes. It has been known in the art that chlorine containing chemicals such as Cl.sub.2, Cl.sub.2 O, ClO.sub.2, NaClO and NaClO.sub.3 can be used in the selective delignification of vegetable materials. The delignification of vegetable materials with the above-mentioned chlorine containing chemicals in combination with the extraction treatment with an organic solvent such as monoethanol amine or NaOH has been described in Japanese Pat. No. 573,946 issued to Ayukawa (published Nov. 26, 1969) and in the report by Norman S. Tompson et al, appearing in "TAPPI" Vol. 47, No. 3, pages 157-162 (March 1964), for example. Although these delignification processes have advantages in that they produce pulp which is relatively free of chemical and/or mechanical damage with a relatively high yield, since no economical and effective recovery technology for treating waste liquor generated in the pulping step has been established, such pulping processes have not yet been practiced on a mill scale.